Low pressure pumping



April 2, 1963 R. B. GODDARD LOW PRESSURE PUMPING Filed Jan. 19, 1959 FIG.

INVENTOR. ROBERT B. GODDARD BY ATTOR Y Sttes te 3,084,257 Patented Apr. 2, 1963 tice 3,084,257 LOW PRESSURE PUMPING Robert B. Goddard, Cambridge, Mass, assignor, by mesne assignments, to Laboratory for Electronics, Inc,

Boston, Mass, a corporation of Deiaware Filed Jan. 19, 1959, Ser. No. 787,681 2 Claims. (Cl. 250-845) This invention relates in general to low pressure gas pumps and, in particular, to a hydrogen or hydrogen isotope pump specially adapted for use with neutron generating tubes in closely confined areas.

Neutron generators capable of producing a high intensity, controllable neutron flux have many applications, but perhaps the most frequent utilization occurs in oil well logging systems. In this application a neutron generator together with associated detecting equipment is lowered in a cartridge type of unit down an oil well bore and the information recorded in the detecting equipment is taken to be indicative of the geological composition of the substrata.

The use of neutron generator tubes in oil well logging imposes severe limitations on the physical nature of the equipment. It is apparent that in logging oil well strata the neutron generator and its associated equipment must be housed within a package of relatively small dimensions adapted to withstand the extremes of pressure and temperature encountered. For example, an oil well might well have an inner casing diameter 5 /2 inches and extend to a depth on the order of 20,000 feet. In these circumstances, the maximum diameter of the cartridge containing the neutron generator would be limited to approximately 4 inches. Since the device is often lowered to the bottom of the well at speeds as high as feet per second, collisions with irregularities in the well casing produce severe mechanical shock which must be absorbed to avoid costly shutdowns and reruns.

Neutron generator tubes capable of performing under these conditions have been constructed, a typical example being that described in a co-pend-ing application Serial No. 673,044, now US. Patent 2,994,777 issued August 1, 1961. These generator tubes employ one or more of the hydrogen isotopes and generate neutrons from the DD, T-D, or DT reactions. In order to achieve stable neutron output over long periods of time, it is necessary to maintain a flow of the hydrogen isotope through the tube at constant pressure. If the flow rate is decreased below a critical pressure, additional power is consumed accompanied by a decrease in neutron yield. In order to maintain a dynamic pressure equilibrium, it is desirable to admit the hydrogen gas at a given rate and pump it out at this same rate. For most such neutron generator tubes, the desired equilibrium pressure is on the order of a few microns.

The problems of procuring a gas pump to maintain this flow under the conditions imposed in oil well logging units are very diflicult and well recognized. Thus the space limitations require a gas pump of very small size, and the power limitations indicate a pump which operates on very little power. One type of pump which is small enough for this application is known as the hot wire ump, in which pumping action is obtained by heating a metal filament; the evaporated ions then trapping the vapor against the walls of the tube in a getter type of action. This type of pump has the drawback of requiring rather large amounts of power and further of having rather limited life. For example, one such pump requires 50 watts of power and has a useful operating life of only 50 hours.

With the foregoing limitations of the art in view, it is, therefore, a primary object of this invention to provide a hydrogen pump of small size which requires no external power source and which will operate for extended periods of time.

It is another object of this invention to provide a small efiicient pump for hydrogen isotopes which has no moving parts and requires no applied power.

In general, the pump of this invention comprises a container having a narrow inlet opening, the container being partially filled with outgassed titanium powder. The pumping elfect is derived from the sorption characteristics of the titanium powder at temperatures as low as 20 degrees Centigrade. Stability of this pump is provided by the inlet stricture which governs the pressure relationship between the pump volume and the volume of the generator tube being pumped. By providing a suitable amount of titanium and controlling the rate of gas flow into the neutron generator tube, a pump of this type can be made to maintain dynamic pressures in the micron region for exceedingly long periods of time.

These and other objects and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawing in which:

FIG. 1 is an illustration partly in perspective and partly in diagrammatic form of a typical neutron generator within an oil logging tube; and

FIG. 2 is an illustration in perspective view of a hydrogen pump embodying the principles of this invention.

With specific reference now to FIG. 1, the neutron generator is shown enclosed by a broken line 10 representing the general configuration of a well surveying tool within a typical oil well bore. The purpose of such illustration is generally to indicate the problem encountered in devising the auxiliary equipment of such a generator to fit within a container no more than three or four inches in diameter.

More specifically, with reference to FIG. 1, the tool is seen to contain the neutron generator tube 11, power source 12, a gas reservoir 14, emitter chamber 15, and hydrogen pump 16. Electrical power must, of course, be supplied from the earths surface, and is transmitted through cable 17. As indicated previously, in order to I provide a constant neutron flux from the generator, a constant how of hydrogen isotope must be maintained through the generator tube after the tube is first fully evacuated by conventional means (not shown). This is accomplished by allowing hydrogen isotopes stored in reservoir =14 to pass into generator tube 11 through emitter chamber 15. This emitter chamber may be the conventional platinum leak apparatus which diffuses hydrogen at a known rate, or a heated hollow filament in which the leak rate for hydrogen is a function of the applied current. The detailed construction features of pump #16 itself will be described below, noting, however, that it must provide a pump rate equal to the emitter leak rate in order to mm'ntain the required constant flow of hydrogen.

Turning now to FIG. 2, the pump of this invention is seen to comprise a hollow container 23, which may be formed of glass, metal, or any other material which can provide a gas-tight volume, having a constricted inlet passage 22, which passage contains a porous plug 23 of glass wool or other inert filtering material, and communicates with the interior of the neutron generator tube 11. A mass of titanium powder 24, which has previously been outgassed, is included within container 21. The amount of titanium powder required depends on the desired characteristics of the pump in a manner which will be described below.

As indicated previously, the pumping action is accomplished by the sorption of hydrogen or hydrogen isotope on the titanium metal. This process is essentially monomolecular. Hence the duration or life of the pumping action for a given quantity of titanium and with a given flow of hydrogen can be calculated from the relation:

moles of titanium moles per hour of hydrogen For a given flow rate the pumping rate must be constant to maintain constant pressure. Since the pumping rate in this process is proportional to the number of titanium atoms present, then,

Where N equals the number of titanium atoms which have not sorbed hydrogen, k=probability of a titanium atom sorbing hydrogen at a given pressure.

Integrating the above expression, the equation:

is obtained where N is the number of atoms of titanium with no hydrogen sorbed at time zero. For small values of kt this is approximately a linear relation and since the pressure is proportional to the number of titanium atoms present which do not have hydrogen sorbed on them, a five percent increase in pressure would be observed during the first five percent of the life of the pump.

Such a pump has been constructed with a glass 5 milliliter flask and used to pump a neutron generator tube having a volume of 200 cubic centimeters. Five grams of titanium powder were enclosed in the flask and the hydrogen flow rate was 200 micron cubic centimeters per second at a total pressure of a few microns. Under these conditions the life of the pump would be 2300 hours, no power source being required. Five percent of the theoretical life would then be 115 hours. This pump was operated for a period of 80 hours with no observable changes in pressure.

The expected life of a pump of this type can be extended by using more titanium powder if the pressure within the generator tube can be made independent of changes in pressure within the pump jvolume. lI'he flow of hydrogen through the constriction is proportional to the pressure in the neutron generator tube minus the pressure in the'pump. The pressure differential is dependent upon the impedance of the constriction and if this impedance is made high so that the pressure within'the pump'volume is very much smaller than that Withinthe neutron generator tube, the [HOW of gas through'the inlet is essentially independent of the pressure within the pump volume, providing only that it remain much smaller than the pressure within the'neutron generator tube.

The titanium used in the pump referred to above was prepared by heating powdered titanium hydride at a temperature of about 400 C. ina vacuum and pumping ofi the'evolved hydrogen gas. rAnothermethodwhich has been used successfully is to'v'acuumevaporate titanium onto the interior wall of the pump. I Any method of pre- =duration (in hours) paring outgassed pure titanium will, however, be satisfactory.

While the invention herein has been described specifically in terms of a pump for a neutron generator tube, it should not be so limited, but rather in view of the fact that numerous modifications and departures may now be made by those skilled in the art, it should be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

'1. Neutron generating apparatus comprising a neutron generator tube wherein neutrons are produced by means of a gas discharge reaction at low pressures of hydrogen isotope, a source of potential or applying suflicient voltage across said tube, a gas reservoir for storage of said hydrogen isotope communicating with the interior of said discharge tube, a gas emitter control disposed between said reservoir and said tube for controlling the amount of: hydrogen isotope entering said tube, a gas tight container having outgassed titanium metal disopsed therein communicating with the interior of said discharge tube, said titanium thereby serving to pump hydrogen isotope gas from the interior of said discharge tube for maintaining a substantially constant low pressure therein without the addition of energy to said titanium metal.

2. A neutron generator system including a discharge tube having opposed electrodes wherein neutrons are produced by means of a gas discharge reaction between hydrogen isotopes at low pressure, a source of electrical potential applied across said electrodes for initiating and sustaining said discharge, a gas reservoir containing gaseous hydrogen isotope communicating with the interior of said discharge tube through a first gas tight tubing for replenishing said gaseous hydrogen isotope, a gas emitter control interposed between said reservoir and said discharge tube for controlling the flow of said gaseous hydrogen isotopes from said reservoir, a hydrogen pump communicatingwith the interior of said discharge tube by means of a second gas tight tubing, said hydrogen pump comprising a gas tight container having a constricted inlet orifice connected with said second tubing, and titanium metal disposed within said container, said titanium metal being in an initially outgassed condition,

'said titanium -metal being operative to sorb gas from said reservoir without the application of energy to said titanium metal.

References Cited in the file o'fthis patent UNITED STATES PATENTS 1,289,521 Newcomb Dec. 31, 1918 2,397,764 Stu-bbs Apr. 2, 1946 2,636,664 Hertzler Apr. 28, 1953 2,735,019 Dewan Feb. 14, 1956 2,796,555 Connor June 18, 1957 2,841,323 Lindenblad July 1, 1958 2,850,225 Herb Sept. 2, 1958 FOREIGN PATENTS 7 724,441 Great Britain Feb. 23, 1955 

1. NEUTRON GENERATING APPARATUS COMPRISING A NEUTRON GENERATOR TUBE WHEREIN NEUTRONS ARE PRODUCED BY MEANS OF A GAS DISCHARGE REACTION AT LOW PRESSURES OF HYDROGEN ISOTOPE, A SOURCE OF POTENTIAL FOR APPLYING SUFFICIENT VOLTAGE ACROSS SAID TUBE, A GAS RESERVIOR FOR STORAGE OF SAID HYDROGEN ISOTOPE COMMUNICATING WITH THE INTERIOR OF SAID DISCHARGE TUBE, A GAS EMITTER CONTROL DISPOSED BETWEEN SAID RESERVOIR AND SAID TUBE FOR CONTROLLING THE AMOUNT 